High-strength twip steel sheet and method of manufacturing the same

ABSTRACT

The present invention features a high-strength and light TWIP steel sheet which can be used to manufacture vehicle body parts, and a method of manufacturing the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(a) priority to KoreanApplication No 10-2009-0032093, filed on Apr. 14, 2009, the disclosureof which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a twinning induced plasticity (TWIP)steel sheet having high strength and high workability and a method ofmanufacturing the same. In particular embodiments, the present inventionrelates to a high-strength TWIP steel sheet having excellent yieldstrength and elongation rate and a method of manufacturing the same.

2. Description of the Related Art

Generally, a high-strength steel sheet that has excellent formability,and which can decrease the weight of a vehicle in order to increase thevehicle fuel efficiency and prevent air pollution, is a requirement inthe field of vehicle steel sheets.

In particular, when complicated vehicle body components are manufacturedusing the superior formability of a steel sheet material, technologiesfor manufacturing a twinning induced plasticity (TWIP) steel sheethaving high strength have been proposed.

However, in the conventional technologies, although a high-tension steelsheet is required to increase the strength of a vehicle body, it cannotbe formed into a complicated part because it has a low elongation rate,so that it is formed into several parts, and then the several parts arewelded together to obtain a final product. Further, when a collisionmember is manufactured through the conventional technologies, since theconventional TWIP steel sheet has a yield strength of about 400 MPa,which is lower than that of a transformation induced plasticity (TRIP)steel sheet or dual phase (DP) steel sheet having a tensile strength ofabout 980 MPa corresponding to the yield strength of about 400 MPa, theinitial collision performance is low.

Further, although the conventional TWIP steel sheet must be suitablyhighly pressurized at the time of cold rolling in order to increase itsyield strength, concomitant problems, such as fracture, side crackingand the like, occur because it has the characteristics ofultrahigh-tension steel. As a result it is difficult to economicallyobtain products.

The above information disclosed in this the Background section is onlyfor enhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a high-strength TWIP steelsheet preferably having a yield strength of about 550 MPa and a suitablyimproved elongation rate, and a method of manufacturing the same.

In a preferred embodiment of the invention, in order to accomplish theabove object, an aspect of the present invention preferably provides amethod of manufacturing a high-strength TWIP steel sheet, comprising:cold-rolling a hot-rolled steel sheet having a composition preferablyincluding 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si),15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt %or less of phosphorus (P), 0.001˜0.002 wt % of sulfur (S), 4.0˜5.0 wt %of titanium (Ti), and residual iron and inevitable impurities. Infurther embodiments the method comprises continuously annealing thecold-rolled steel sheet.

In preferred embodiments of the method, the hot-rolled steel sheet maybe suitably obtained by preparing a powdered titanium-manganese (Ti—Mn)alloy, melting the titanium-manganese (Ti—Mn) alloy in a convertertogether with another composition and then preferably continuouslycasting them to form a slab, preferably hot-rolling the slab from1100˜1300° C. to 850˜950° C., and then suitably cooling the hot-rolledslab. In further embodiments, the hot-rolled slab may be suitablyair-cooled from 850˜950° C. to 650˜750° C. at a cooling rate of 35˜45°C./sec.

In another aspect, the present invention preferably provides ahigh-strength TWIP steel sheet having a composition, the compositionpreferably comprising: 0.15˜0.30 wt % of carbon (C); 0.01˜0.03 wt % ofsilicon (Si); 15˜25 wt % of manganese (Mn); 1.2˜3.0 wt % of aluminum(Al); 0.020 wt % or less of phosphorus (P); 0.001˜0.002 wt % of sulfur(S); 4.0˜5.0 wt % of titanium (Ti); and residual iron and inevitableimpurities.

In certain exemplary embodiments of the invention, the composition maybe suitably formed into the TWIP steel sheet having a yield strength of550 MPa or more by suitably preparing a powdered titanium-manganese(Ti—Mn) alloy, suitably melting the titanium-manganese (Ti—Mn) alloy ina converter together with another composition and then continuouslycasting them to preferably form a slab, hot-rolling the slab torecrystallize, cold-rolling the hot-rolled slab, and then suitablyannealing the cold-rolled slab.

Preferably, in the hot rolling, the continuous-cast slab may behot-rolled from 1100˜1300° C. to 850˜950° C., and then the hot-rolledslab is suitably air-cooled from 850˜950° C. to 650˜750° C. at a coolingrate of 35˜45° C./sec.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, Watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum).

As referred to herein, a hybrid vehicle is a vehicle that has two ormore sources of power, for example both gasoline-powered andelectric-powered.

The above features and advantages of the present invention will beapparent from or are set forth in more detail in the accompanyingdrawings, which are incorporated in and form a part of thisspecification, and the following Detailed Description, which togetherserve to explain by way of example the principles of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a graph showing the state in which crystal grains haverecrystallized through heat treatment; and

FIG. 2 is a graph showing the change in yield strength of a TWIP steelsheet of the present invention compared with that of a conventionalsteel sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described herein, the present invention includes a method ofmanufacturing a high-strength TWIP steel sheet, comprising cold-rollinga hot-rolled steel sheet having a composition comprising carbon (C),silicon (Si), manganese (Mn), aluminum (Al), phosphorus (P), sulfur (S),titanium (Ti), and residual iron and inevitable impurities, andannealing the cold-rolled steel sheet.

In one embodiment, the hot-rolled steel sheet has a compositioncomprising 0.15˜0.30 wt % of carbon (C).

In another embodiment, the hot-rolled steel sheet has a compositioncomprising 0.01˜0.03 wt % of silicon (Si).

In a further embodiment, the hot-rolled steel sheet has a compositioncomprising 15˜25 wt % of manganese (Mn).

In one embodiment, the hot-rolled steel sheet has a compositioncomprising 1.2˜3.0 wt % of aluminum (Al).

In another embodiment, the hot-rolled steel sheet has a compositioncomprising 0.020 wt % or less of phosphorus (P).

In a further embodiment, the hot-rolled steel sheet has a compositioncomprising 0.001˜0.002 wt % of sulfur (S).

In one embodiment, the hot-rolled steel sheet has a compositioncomprising 4.0˜5.0 wt % of titanium (Ti).

In another embodiment, the hot-rolled steel sheet has a compositionfurther comprising and residual iron and inevitable impurities.

In a further embodiment, annealing the cold-rolled steel sheet isperformed continuously.

In another aspect, the invention features a high-strength TWIP steelsheet having a composition, the composition comprising carbon (C);silicon (Si); manganese (Mn); aluminum (Al); phosphorus (P); sulfur (S);titanium (Ti); and residual iron and inevitable impurities.

In another aspect, the invention features a high-strength TWIP steelsheet having a composition, the composition comprising 0.15˜0.30 wt % ofcarbon (C); 0.01˜0.03 wt % of silicon (Si); 15˜25 wt % of manganese(Mn); 1.2˜3.0 wt % of aluminum (Al); 0.020 wt % or less of phosphorus(P); 0.001˜0.002 wt % of sulfur (S); 4.0˜5.0 wt % of titanium (Ti); andresidual iron and inevitable impurities.

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the attached drawings.

The present invention preferably provides a method of manufacturing ahigh-strength TWIP steel sheet having a suitable yield strength of about550 MPa and an improved elongation rate.

In particular preferred embodiments, in the present invention, in orderto suitably improve the yield strength of the TWIP steel sheet bypreventing the formation of twins, titanium, which is preferably presentin a crystal grain in the form of a precipitate and serves to suitablyprevent twins from being formed in the early stage, may preferably beconsidered. Titanium is an element having high oxidizability and is ametal having a high boiling point, and when it is added at the time ofrefining steel, there is a danger that it is oxidized into titaniumoxides (TiO_(x)) and thus in certain examples the titanium oxides(TiO_(x)) are present in the steel as impurities. In certainembodiments, in the case of TWIP steel, since many kinds of elements areadded to the TWIP steel, the melting point of the TWIP steel is suitablylowered, so that there are many difficulties in adding titanium (Ti) tothe TWIP steel. Furthermore, when temperature control is not properlyconducted at the time of cooling steel after hot-rolling the steel, aprecipitate, called “Ti₃C₈”, is suitably formed. This precipitate ispreferably present in a crystal grain boundary and suitably causes thedeterioration of formability, such as, but not limited to, elongationrate, plastic deformation ratio or the like.

Accordingly, the present invention features, in preferred embodiments,two methods of suitably applying titanium to a TWIP steel sheet. In afirst embodiment, interstitial free (IF) steel to which titanium (Ti) ispreferably added through a low-cost process is suitably melted in avacuum furnace, and then carbon and other components are added to themelted IF steel. Preferably, in this method, the oxidization of thetitanium (Ti) added to the IF steel can be suitably prevented, andtitanium (Ti) is not required to be further added thereto, so that thismethod may be a suitably economical method.

In a second embodiment, a powdered titanium-manganese (Ti—Mn) alloy ispreferably prepared and then suitably added to steel at the time ofrefining the steel. Preferably, when the steel is made in the form of ametal matrix composite (MMC), the steel can have a desired componentratio without being influenced by its composition. In further preferredembodiments, when this MMC steel is suitably charged into the vacuumfurnace, it is easily melted because the titanium-manganese (Ti—Mn)alloy powder has high surface energy (large surface area). Accordingly,in further related embodiments, it is suitably easily supercooled. Thismethod is advantageous in terms of component control.

In certain preferred embodiments of the present invention, the twomethods were suitably employed, and final components and their materialproperties did not suitably differ from each other.

In other preferred embodiments of the invention, in order to suitablymanufacture a high-strength TWIP steel sheet, the added amounts ofmanganese (Mn), carbon (C) and aluminum (Al) must be suitably adjustedand the microstructure thereof must be suitably controlled because thecharacteristics of TWIP steel are preferably exhibited in an austenitesingle phase. In further embodiments, in order to obtain a completeaustenite single phase, the amounts of manganese (Mn) and carbon (C),which are elements stabilizing austenite, must be suitably optimized.Preferably, when twins are formed through the optimization of theamounts of manganese (Mn) and carbon (C), the twin formation rate mustbe suitably controlled by aluminum (Al). Moreover, in furtherembodiments, in order to increase the yield strength of the TWIP steelsheet, the amount of titanium must be suitably optimized.

In further preferred embodiments of the invention, in order to suitablymanufacture a TWIP steel sheet having improved yield strength andelongation rate while utilizing the characteristics of the constituentsthereof, these constituents are preferably required to be combined witheach other in a suitably appropriate combination ratio. Accordingly, thepresent inventors have discovered the appropriate combination ratiothrough many trials and errors, and, as a result, that the presentinvention demonstrated that the following composition ratio isappropriate according to the preferred embodiments of the presentinvention. The results thereof are given in Table 1.

TABLE 1 Components C Si Mn Al P S Ti Fe Contents 0.15~0.30 0.01~0.0315.0~25.0 1.20~3.00 0.020 0.001~0.002 4.00~5.00 balance (wt %) or less

According to certain preferred embodiments, the reasons for thenumerical ranges of the composition of the TWIP steel sheet aredescribed as follows)

(i) Carbon (C) 0.15˜0.30 wt %

According to preferred embodiments of the invention, since carbon (C)contributes to the stabilization of an austenite phase, it isadvantageous that the content thereof suitably increases. Preferably,when the content thereof is less than 0.15 wt %, a′-martensite issuitably formed at the time of deforming a TWIP steel sheet, andmachining cracks are also suitably formed, thereby deteriorating theductility of the TWIP steel sheet. In other embodiments, when thecontent thereof is more than 0.30 wt %, the stability of the austenitephase is suitably increased, and thus the transition of deformationmechanism of the TWIP steel sheet can suitably occur due to slipdeformation. In related embodiments, the reason why the transition ofdeformation mechanism of the TWIP steel sheet occurs is that theformability of the TWIP steel sheet is suitably decreased due to theincrease in laminated defect energy.)

(ii) Silicon (Si) 0.01˜0.03 wt %

According to preferred embodiments of the invention, silicon (Si) is asubstitutional solid solution element and serves to suitably improve thestrength of a material by maintaining the state of a solid solutionstructure to that at room temperature when the material is suitablyheated to a melting point or higher and then cooled. Preferably, whenthe content of silicon (Si) is preferably less than 0.01 wt %, soluteeffect is suitably decreased, and thus it is difficult to suitablyimprove the strength of the material. In other certain embodiments ofthe invention, when the content thereof is preferably more than 0.03 w%, many defects can be caused at the time of welding.

(iii) Manganese (Mn) 15˜25 wt %

According to preferred embodiments of the invention, like carbon (C),manganese (Mn) is an element for suitably stabilizing austenite.Preferably, manganese (Mn) forms a′-martensite worsening formabilitywhen the content of manganese (Mn) is less than 15 wt %. In furtherembodiments, it is verified that the strength of the TWIP steel sheet issuitably increased, but it cannot be expected that the TWIP steel sheethas high ductility, which is a characteristic of TWIP steel.

In related embodiments, the content of manganese (Mn) is preferablylimited to a lower limit of 15 wt %. Preferably, the formation of twinsis prevented even when the content thereof is more than 25 wt %. Thisphenomenon is directly related to the deterioration of ductility. Infurther embodiments, as the content thereof is suitably increased, theTWIP steel sheet is easily cracked at the time of hot rolling, and theproduction cost of the TWIP steel sheet is suitably increased due to theaddition of expensive elements, so that it is preferred that the contentthereof be preferably limited to 25 wt %.

Accordingly, in further embodiments, the production cost of the TWIPsteel sheet can be suitably decreased by appropriately adjusting thecontent of carbon (C) such that the content of manganese (Mn), which isan expensive element, is suitably minimized, and the deformation oftwins can be suitably induced by adjusting the content of carbon (C) andaluminum (Al).

(iv) Aluminum (Al) 1.2˜3.0 wt %

According to further preferred embodiments of the invention, andgenerally, aluminum (Al) is preferably added in order to suitablydeoxidize the TWIP steel sheet. IN certain preferred embodiments of thepresent invention, it is related to suitably increasing the ductility ofthe TWIP steel sheet preferably, unlike carbon (C) or manganese (Mn),aluminum (Al) is an element for preferably stabilizing ferrite, butserves to suitably increase the ductility of the TWIP steel sheet byincreasing laminated defect energy and preventing the formation ofe-martensite. In further embodiments, aluminum (Al) contributes to theminimization of the content of manganese (Mn) because it preferablyprevents the suitable formation of e-martensite even when the content ofmanganese (Mn) is approximate to a lower limit of 15 wt %. Preferably,when the content of aluminum (Al) is suitably less than 1.2 wt %, theductility of the TWIP steel sheet is decreased regardless of theincrease in strength thereof because e-martensite is formed. In otherpreferred embodiments, when the content of aluminum (Al) is preferablymore than 3.0 wt %, the ductility of the TWIP steel sheet is suitablydecreased by preventing the formation of twins, and the surface qualityof the TWIP steel sheet is suitably deteriorated by decreasingcontinuous castability and causing the oxidization of the hot-rolledTWIP steel sheet.

(v) Phosphorus (P) 0.020 wt % or Less, Sulfur (S) 0.001˜0.002 wt %

According to preferred embodiments of the invention, although phosphorus(P) is an element which is preferably added at the time of manufacturingsteel, it suitably causes segregation, thus suitably decreasingworkability. Therefore, according to certain embodiments of theinvention, it is advantageous that the content of phosphorus (P) besuitably low, and it is preferred that the content of phosphorus (P) bepreferably limited to 0.020 wt % or less.

According to further embodiments, since sulfur (S) causes cracks byforming coarse manganese sulfide (MnS) and decreases hole expandability,in certain preferred embodiments it must be suitably suppressed.Preferably, like phosphorous (P), it is advantageous for the content ofsulfur (S) to be suitably low, and it is preferred that the content ofsulfur (S) be preferably limited to 0.001˜0.002 wt %.

(vi) Titanium (Ti) 4.0˜5.0 wt %

According to further preferred embodiments of the invention, titanium(Ti), which is present in crystal grains in the form of a precipitate,serves to suitably prevent the formation of twins in the initial stageand functions to improve the yield strength of the TWIP steel sheet byincreasing the initial deformation resistance of the TWIP steel sheet,that is, by preventing the formation of the twins. Accordingly, due tothe addition of titanium (Ti), crystal grains can be suitablyminiaturized and precipitates can be formed in the crystal grains or ata crystal grain boundary. The optimal titanium content ratios accordingto certain preferred embodiments of the invention are given in Table 2below.

As shown in Table 2, Examples 1 to 12 show the mechanical properties ofthe TWIP steel sheet including 4˜5 wt % of titanium, ComparativeExamples 1 to 16 show the mechanical properties of the TWIP steel sheetincluding less than 4 wt % of titanium, and Comparative Examples 17 to18 show the mechanical properties of the TWIP steel sheet including morethan 5 wt % of titanium. As shown here and according to certainpreferred embodiments of the invention, the mechanical properties maypreferably include, but are not limited to, yield strength, which is amajor object of the present invention, tensile strength, and elongationrate.

In these Examples and Comparative Examples, a slab preferably preparedthrough continuous casting using a converter was suitably hot-rolledfrom 1300° C. to 900° C. and then suitably air-cooled from 900° C. to700° C. at a cooling rate of 40° C./sec to form a hot-rolled coil, andthen the hot-rolled coil was cold-rolled seven times.

In further related embodiments, the cold-rolled coil was suitablyheat-treated at a temperature of 700˜850° C. for 5 minutes using acontinuous annealing furnace to recrystallize crystal grains and thus torecover elongation rate. According to further related embodiments, thefact that the crystal grains were recrystallized and the elongation ratewas recovered can be seen from FIG. 1. Referring to FIG. 1, it can beseen that the recrystallization of the crystal grains was suitablycompleted after 180 seconds.

TABLE 2 Average Yield Tensile Elongation crystal Ti Mn Al strengthstrength rate grain size (wt %) (wt %) (wt %) (MPa) (MPa) (%) (m) Exp. 14.00 15.00 1.2 561 987 63.6 2.1 Exp. 2 4.50 15.00 1.2 570 983 61.0 2.3Exp. 3 5.00 15.00 1.2 577 994 61.0 2.5 Exp. 4 4.00 25.00 1.2 559 101969.3 2.9 Exp. 5 4.50 25.00 1.2 588 1009 68.5 2.0 Exp. 6 5.00 25.00 1.2587 1062 63.2 2.12 Exp. 7 4.00 15.00 3.0 551 983 67.1 2.6 Exp. 8 4.5015.00 3.0 550 998 66.8 2.88 Exp. 9 5.00 15.00 3.0 552 998 68.4 2.66 Exp.10 4.00 25.00 3.0 601 1018 60.3 2.51 Exp. 11 4.50 25.00 3.0 561 997 63.62.6 Exp. 12 5.00 25.00 3.0 559 1014 66.7 2.3 Comp. Exp. 1 0.00 15.00 1.2410 978 42.3 6.83 Comp. Exp. 2 0.00 25.00 1.2 402 978 47.0 9.35 Comp.Exp. 3 0.00 15.00 3.0 490 950 48.2 12.1 Comp. Exp. 4 0.00 25.00 3.0 505980 47.4 7.0 Comp. Exp. 5 1.50 15.00 1.2 493 960 42.3 11.1 Comp. Exp. 61.50 25.00 1.2 462 963 48.2 12.4 Comp. Exp. 7 1.50 15.00 3.0 499 94247.1 8.3 Comp. Exp. 8 1.50 25.00 3.0 493 931 51.3 9.2 Comp. Exp. 9 3.0015.00 1.2 460 922 54.1 12.4 Comp. Exp. 10 3.00 25.00 1.2 430 980 56.33.3 Comp. Exp. 11 3.00 15.00 3.0 410 977 66.2 4.2 Comp. Exp. 12 3.0025.00 3.0 423 977 63.0 3.5 Comp. Exp. 13 3.88 15.00 1.2 469 963 66.7 3.9Comp. Exp. 14 3.88 25.00 1.2 410 977 68.9 3.8 Comp. Exp. 15 3.88 15.003.0 403 973 68.3 4.1 Comp. Exp. 16 3.88 25.00 3.0 411 974 69.2 3.9 Comp.Exp. 17 5.15 15.00 1.2 512 958 58.3 3.7 Comp. Exp. 18 5.15 25.00 1.2hot-rolled crack

Referring to Table 2, in Examples 1 to 6, a TWIP steel sheet includes1.2 wt % of aluminum, 15˜25 wt % of manganese, and 4˜5 wt % of titanium.Preferably, the yield strength of the TWIP steel sheet of Examples 1 to6 is in a range of 561˜588 MPa, which is suitably increased by 185 MPacompared to that of the TWIP steel sheet of the following ComparativeExamples. In further embodiments, the elongate rate of the TWIP steelsheet of Examples 1 to 6 is preferably in a range of 61˜69.3 wt %, whichis also suitably increased compared to that of the steel sheet of thefollowing Comparative Examples.

Preferably, in Examples 7 to 12, a TWIP steel sheet includes 3.0 wt % ofaluminum, 15˜25 wt % of manganese, and 4˜5 wt % of titanium. Accordingto further preferred embodiments, the yield strength of the TWIP steelsheet of Examples 7 to 12 is in a range of 550˜601 MPa, which issuitably increased by 201 MPa compared to that of the TWIP steel sheetof the following Comparative Examples. Further, the elongate rate of theTWIP steel sheet of Examples 7 to 12 is in a range of 60.3˜68.4 wt %,which is also increased compared to that of the steel sheet of thefollowing Comparative Examples.

Generally, as yield strength is suitably increased, elongation rate isdecreased. However, in preferred embodiments of the present invention,it was found that crystal grains suitably formed by the addition oftitanium is miniaturized, thus suitably increasing the elongation rate.Accordingly, in preferred embodiments of the present invention, it wasdetermined that the yield strength is improved by delaying the formationof initial twins, the delay being caused by titanium precipitates.

In Comparative Examples 1 to 16, a TWIP steel sheet includes 1.2˜3.0 wt% of aluminum, 15˜25 wt % of manganese, and less than 4 wt % oftitanium. It can be seen that the yield strength of the TWIP steel sheetof Comparative Examples 1 to 16 is increased to 499 MPa, which is lowcompared to the yield strength (500 Mpa) of a conventional TWIP steelsheet or dual phase steel sheet. From this result, it can be seen thatthe content of titanium is suitably determined depending on theappropriate conditions.

In Comparative Examples 17 to 18, a TWIP steel sheet includes 1.2 wt %of aluminum, 15˜25 wt % of manganese, and more than 4 wt % of titanium.From Comparative Example 17, it can be seen that the yield strength ofthe TWIP steel sheet of Comparative Example 17 is increased to themaximum yield strength and is then decreased to about 500 MPa, and thatthe elongation rate and tensile strength thereof is decreased. Thereason for this has been found to be that titanium precipitates arecoarse.

In Comparative Example 18, since side cracks occur during hot rollingwhen the content of manganese becomes 25 wt %, which is the maximumvalue of the conventional TWIP steel sheet, a final product cannot beproduced. According to preferred embodiments of the invention, thereason for this was determined to be that the strength of a hot-rolledsteel sheet is suitably increased according to the increase in contentof titanium and manganese.

For example, in certain embodiments of the invention, when titanium isadded in an amount of 4 wt % or more, the mechanical properties of theTWIP steel sheet are rapidly improved. The titanium carbide (TiC)precipitates formed in grains or grain boundaries cause increase inyield strength by preventing the movement of initial dislocation, thatis, by delaying the formation of twins, but allows elongation rate to bemaintained in a plastic region because they do not prevent the formationof twins in the plastic region. As a result, when alloying processes areused to manufacture, the TWIP steel sheet according to the presentinvention, a high-strength steel strip or sheet which can be cold-formedsuch that it is suitable to manufacture a vehicle body panel and whichis light can be obtained.

Therefore, according to preferred embodiments of the invention, it ispreferred that the optimum content of titanium be in a range of 4˜5 wt%.

Accordingly, the high-strength TWIP steel sheet has a preferredcomposition including 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % ofsilicon (Si), 15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum(Al), 0.020 wt % or less of phosphorus (P), 0.001˜0.002 wt % of sulfur(S), 4.0˜5.0 wt % of titanium (Ti), and residual iron and inevitableimpurities.

According to further preferred embodiments of the invention, thecomposition is hot-rolled, coiled, cold-rolled, and then continuouslyannealed to manufacture a high-strength TWIP steel sheet having asuitably improved yield strength and excellent elongation rate. Apreferred method of manufacturing high-strength TWIP steel sheetaccording to preferred embodiments of the present invention is describedas follows.

In preferred embodiments in the method of manufacturing thehigh-strength TWIP steel sheet, the preferred composition including0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si), 15 25 wt %of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt % or less ofphosphorus (P), 0.001˜0.002 wt % of sulfur (S), 4.0˜5.0 wt % of titanium(Ti) and residual iron and inevitable impurities is suitably melted in aconverter and then continuously cast, and then the cast is suitablyhot-rolled, preferably from 1100˜1300° C. to 850˜950° C. and thenair-cooled from 850˜950° C. to 650˜750° C. at a cooling rate of 35˜45°C./sec to suitably obtain the high-strength TWIP steel sheet.

Preferably, when the hot rolling of the cast is suitably completed in apreferred range of 850˜950° C., the formation of Ti₃C₈ can be suitablyprevented, but when it is completed at about 800° C., Ti₃C₈ is suitablyformed instead of TiC, and thus the formability of the TWIP steel sheetcan be suitably deteriorated. In further embodiments, when thehot-rolled cast is preferably air-cooled to a coiling temperature, areason why the cooling rate is preferably maintained at 35˜45° C./sec isthat this cooling rate is a suitably optimal cooling rate at whichprecipitates are suitably uniformly distributed in crystal grainboundaries. Preferably, in further preferred embodiments, when thecooling rate is preferably more than 45° C./sec, added elements arepresent in the form of a solid solution phase in which they areoversaturated in crystals, so that they are suitably reprecipitatedduring subsequent cold-rolling and annealing processes, with the resultthat unexpected material properties changes, for example, increase ofelongation rate and the like, can occur. In other certain embodiments,when the cooling rate is less than 35° C./sec, crystal grains of ahot-rolled plate are excessively grown, so that the size of crystalgrain of a final plate can be two fold or more of that of a generalmetal. Accordingly, surface defects, such as orange peel and the like,can suitably occur at the time of forming vehicle parts.

Hereinafter, the method of manufacturing the high-strength TWIP steelsheet will be described in detail.

In a preferred embodiment, the above-mentioned composition is suitablymelted in a converter and then suitably continuously cast. Preferably,interstitial free (IF) steel to which titanium (Ti) is added through alow-cost process is suitably melted in a vacuum furnace, and then carbonand other components are preferably added to the melted IF steel,thereby preventing the oxidation of the titanium (Ti) added to the IFsteel. In further embodiments, a powdered titanium-manganese (Ti—Mn)alloy is preferably, prepared and then added to the composition at thetime of refining, so that the composition can have a suitably desiredcomponent ratio without being influenced by its constituents.

Accordingly, the reason for this according to preferred embodiments ofthe invention, is that titanium can be suitably oxidized to titaniumoxides, which are, impurities, when it is preferably added at the timeof refining because it is an element having strong oxidizability andhigh melting point, and that, in the case of TWIP steel, it is suitablydifficult to add titanium thereto because the melting point of the TWIPsteel is suitably lowered due to the addition of many elements.

According to further preferred embodiments, the cast begins to behot-rolled at a temperature that is preferably between 1100˜1300° C.,and the hot rolling of the cast is suitably completed at a temperaturethat is preferably 900° C. In further embodiments, the hot rolled castis suitably air-cooled from 900° C. to 700° C. at a cooling rate of 40°C./sec. Preferably, through these procedures, dislocations accumulatedthrough the hot rolling are suitably released, preferably, entirelyreleased, and thus recrystallization can be completely conducted.According to preferred embodiments of the invention, therecrystallization is necessary to suitably manufacture a high-strengthsteel sheet. Accordingly, in other embodiments, when therecrystallization is not completely conducted, desired rolling reductioncannot be suitably obtained in a subsequent cold rolling process, andthus a final product having desired thickness cannot be suitablyobtained.

According to further preferred embodiments, after the hot rolled cast isair-cooled to 700° C., a cold rolling process is suitably performedthrough a coiling process. Preferably, in the cold rolling process,rolling reduction is suitably maintained at 30˜90%, which may, bepreferably changed depending on the use. In particular preferredembodiments, when the TWIP steel sheet is suitably used to manufacturevehicle parts, it is preferred that the rolling reduction be maintainedat 50˜75%. In other preferred embodiments of the present invention, itmay not be preferred that the rolling reduction be suitably limited topredetermined values.

Preferably, after the cold rolling process, an annealing process issuitably performed. According to further embodiments, in the annealingprocess, the cold-rolled cast is completely recrystallized byheat-treating it, preferably at a temperature of 700˜900° C., andpreferably for 3˜5 minutes, using a continuous annealing furnace. Inrelated embodiments, the recrystallization thereof can be observedthrough its texture photograph and hardness analysis, and is suitablythe same as in a conventional process of producing a TWIP steel sheet.

Further, and according to other preferred embodiments, FIG. 2 shows thechange in yield strength of a TWIP steel sheet of the present inventioncompared with that of a conventional steel sheet.

Preferably, as shown in FIG. 2, when a high-strength TWIP steel sheet issuitably manufactured according to the present invention, the yieldstrength of the portion “A” in FIG. 2 is suitably increased by 180 MPacompared to that of conventional TWIP steel sheets, and the elongaterate thereof is suitably increased by 60% or more compared toconventional TWIP steel sheets, so that its collision performance can beeffectively ensured, and complicated vehicle body parts can also beeasily formed, thereby manufacturing a suitably ultrahigh-strength steelsheet for vehicle parts.

As described in the embodiments above, the high-strength TWIP steelsheet according to the present invention is preferably advantageous inthat its yield strength can be suitably increased by 180 MPa and itselongation rate can be suitably increased by 60% or more, compared toconventional TWIP steel sheets, and thus its collision performance canbe effectively ensured.

According to further embodiments, the high-strength TWIP steel sheetaccording to the present invention is advantageous in that, when it ispreferably used as a material for vehicle body parts, complicatedvehicle body parts can also be suitably easily formed, highstrength-high formability characteristics can be suitably ensured, andthe manufacture of light vehicles can be preferably realized.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A method of manufacturing a high-strength TWIP steel sheet,comprising: cold-rolling a hot-rolled steel sheet having a compositionincluding 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si),15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt %or less of phosphorus (P), 0.001˜0.002 wt % of sulfur (S), 4.0˜5.0 wt %of titanium (Ti), and residual iron and inevitable impurities; andcontinuously annealing the cold-rolled steel sheet.
 2. The methodaccording to claim 1, wherein the hot-rolled steel sheet is obtained bypreparing a powdered titanium-manganese (Ti—Mn) alloy, melting thetitanium-manganese (Ti—Mn) alloy in a converter together with anothercomposition and then continuously casting them to form a slab,hot-rolling the slab from 1100˜1300° C. to 850˜950° C., and then coolingthe hot-rolled slab.
 3. The method according to claim 2, wherein thehot-rolled slab is air-cooled from 850˜950° C. to 650˜750° C. at acooling rate of 35˜45° C./sec.
 4. A high-strength TWIP steel sheethaving a composition, the composition comprising: 0.15˜0.30 wt % ofcarbon (C); 0.01˜0.03 wt % of silicon (Si); 15˜25 wt % of manganese(Mn); 1.2˜3.0 wt % of aluminum (Al); 0.020 wt % or less of phosphorus(P); 0.001˜0.002 wt % of sulfur (S); 4.0˜5.0 wt % of titanium (Ti); andresidual iron and inevitable impurities.
 5. The high-strength TWIP steelsheet according to claim 4, wherein the composition is formed into theTWIP steel sheet having a yield strength of 550 MPa or more by preparinga powdered titanium-manganese (Ti—Mn) alloy, melting thetitanium-manganese (Ti—Mn) alloy in a converter together with anothercomposition and then continuously casting them to form a slab,hot-rolling the slab to recrystallize, cold-rolling the hot-rolled slab,and then annealing the cold-rolled slab.
 6. The high-strength TWIP steelsheet according to claim 5, wherein, in the hot rolling, thecontinuous-cast slab is hot-rolled from 1100˜1300° C. to 850˜950° C.;and then the hot-rolled slab is air-cooled from 850˜950° C. to 650˜750°C. at a cooling rate of 35˜45° C./sec.
 7. A method of manufacturing ahigh-strength TWIP steel sheet, comprising: cold-rolling a hot-rolledsteel sheet having a composition comprising carbon (C), silicon (Si),manganese (Mn), aluminum (Al), phosphorus (P), sulfur (S), titanium(Ti), and residual iron and inevitable impurities; and annealing thecold-rolled steel sheet.
 8. The method of manufacturing a high-strengthTWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has acomposition comprising 0.15˜0.30 wt % of carbon (C).
 9. The method ofmanufacturing a high-strength TWIP steel sheet of claim 7, wherein thehot-rolled steel sheet has a composition comprising 0.01˜0.03 wt % ofsilicon (Si).
 10. The method of manufacturing a high-strength TWIP steelsheet of claim 7, wherein the hot-rolled steel sheet has a compositioncomprising 15˜25 wt % of manganese (Mn).
 11. The method of manufacturinga high-strength TWIP steel sheet of claim 7, wherein the hot-rolledsteel sheet has a composition comprising 1.2˜3.0 wt % of aluminum (Al).12. The method of manufacturing a high-strength TWIP steel sheet ofclaim 7, wherein the hot-rolled steel sheet has a composition comprising0.020 wt % or less of phosphorus (P).
 13. The method of manufacturing ahigh-strength TWIP steel sheet of claim 7, wherein the hot-rolled steelsheet has a composition comprising 0.001˜0.002 wt % of sulfur (S). 14.The method of manufacturing a high-strength TWIP steel sheet of claim 7,wherein the hot-rolled steel sheet has a composition comprising 4.0˜5.0wt % of titanium (Ti).
 15. The method of manufacturing a high-strengthTWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has acomposition further comprising residual iron and inevitable impurities.16. The method of manufacturing a high-strength TWIP steel sheet ofclaim 7, wherein annealing the cold-rolled steel sheet is performedcontinuously.
 17. A high-strength TWIP steel sheet having a composition,the composition comprising: carbon (C); silicon (Si); manganese (Mn);aluminum (Al).; phosphorus (P); sulfur (S); titanium (Ti); and residualiron and inevitable impurities.